This invention relates to a spark ignition device for improving fuel ignitability in a direct injection-type internal combustion engine in which fuel is directly injected into combustion chambers.
A spark ignition device capable of achieving improved fuel ignitability by the use of a multiple discharge system has been known. In this system, spark is intermittently discharged two or more times during each combustion cycle of the internal combustion engine.
In recent direct injection-type engines, stratified charge combustion is performed during engine operation under low load condition. That is, a stratified mixture in a form of lumps of sprayed fuel injected from an injector (a fuel injection valve) is moved along the contour of a combustion chamber and by the formation of an intake air stream on the top surface of the piston, thus forming a combustible mixture around a spark plug. It has been generally known that, even within the range of combustible mixture concentration at the plug gap of a spark plug, the combustible mixture in the stratified charge combustion is subject to variations in mixture concentration and ignition time depending upon operating conditions at that time, and that a necessary discharge energy for ignition varies with the mixture concentration.
The amount of discharge energy required in each pulse during aforethe multiple discharge, however, is not fully known. Therefore, the same amount of discharge energy as the amount of single discharge energy like in conventional spark ignition devices is supplied also during multiple discharge even in case of variations in stratified mixture. According to such a conventional method, however, a large amount of discharge energy is supplied even under a very ignitable mixture condition; that is, an excessive discharge energy is supplied. This results in an increase in the amount of electric energy consumed by the ignition system. This requires the use of a large-sized ignition coil and thereby lessens mountability of the spark plug in the internal combustion engine. Furthermore, the increased amount of discharge energy has an adverse effect upon wear resistance of electrodes of the spark plug and upon batteries, alternator, and engine output as well.
In view of the above-described disadvantages inherent in heretofore known spark ignition devices, it is an object of this invention to provide a spark ignition device capable of improving the condition of combustion, controlling discharge energy, and restraining upsizing of the ignition coil by changing the amount of each discharge energy in accordance with operating condition and by supplying the optimum amount of discharge energy in accordance with the condition of air-fuel mixture.
According to this invention, in order to perform multiple discharge under a part or all of the operating conditions of at least the stratified charge combustion in a direct injection-type injection engine, a high voltage is intermittently applied from an ignition coil for more than one time in a short time to generate sparks at a spark plug for more than one time, thereby ensuring reliable ignition in response to changes in the concentration of sprayed fuel supplied. Under operating conditions of other than the stratified charge combustion, and under operating conditions of the stratified charge combustion where multiple discharge is not effected, ignition of sprayed fuel is reliably achieved by generating at least one spark at spark plug electrodes. Reliably igniting the sprayed fuel is ensured by generating sparks to the sprayed fuel at a predetermined timing for a number of times suitable to a part or all of the operating conditions of stratified charge combustion in the direct injection-type engine and other operating conditions.
Since the intermittent multiple discharging period is set to gradually increase, the discharge energy for ignition of sprayed fuel can be preferably accumulated even immediately before the end of multiple discharge.